The present invention relates to the treatment of wastewater or sewage so as to remove oxygen-demanding impurities prior to the release of the wastewater effluent into the environment. More particularly, the present invention relates to a process and an apparatus for reducing caustic demand as organic constituents in an anaerobic process stream are metabolized to methane and carbon dioxide.
A number of industries produce a wastewater solution which is acidic in content. The wastewater solution can be high in organic constituents and is generally unacceptable for disposition in a lake or river because of the high oxygen demand associated with the waste. Environmental laws and regulations place limits on the amount of chemical and biological oxygen-demanding substances that can be present in wastewater before such water is allowed to flow into public waterways.
One method of removing chemical and biological oxygen-demanding substances, is through metabolic degradation of the substances by bacteria in aerobic and anaerobic processes. The present invention pertains primarily to anaerobic processes with special application to acidic wastewaters of limited solubility.
In a typical anaerobic filter process, wastewater is directed through an anaerobic reactor wherein microorganisms within an anaerobic medium metabolize the organic substrate in the waste and produce methane and carbon dioxide. Typically, the reactor is a substantially closed containment vessel adapted to receive a wastewater stream. The reactor may contain a packing material to which microorganisms may affix. The packing media may be random, such as pall rings, or an oriented packing, such as packing material marketed under the trademarks Vinyl Core and Koro-Z.
Generally, reactor designs can be divided into two types based on the flow characteristics of the contained liquid. In an upflow reactor, wastewater generally enters from a lower bottom area of a closed reaction vessel and moves upward. Methane and carbon dioxide gases also move upward with the liquid and partition from the liquid phase into the gas phase as the gases approach the surface as a function of pressure differential and partition coefficients. Finished liquid is removed from the top of the liquid level. The top of the vessel defines a closed hood area substantially free of liquid for the collection of the gases.
In a downflow reactor, wastewater generally enters from the top of the reactor and moves downward. Methane and carbon dioxide gas move against the liquid current in an upward movement. However, the movement of the liquid in a downward direction and the ever increasing pressure as depth increases tends to retain soluble gases in the liquid to form an acid buffered liquor. Again, the top of the vessel defines a closed hood area substantially free of liquid for the collection of gases.
In both upflow and downflow systems, effluent leaving the anaerobic reactor is passed through a degassifier and into a settling tank. Methane gas is recovered and is used as an energy source. Sludge produced in the process falls to the bottom of a settling tank and is either recirculated, or further processed for final disposition.
Normally, when the reactor is in operation, the anaerobic process produces carbon dioxide. The carbon dioxide produced by the anaerobic reaction creates a partial pressure which causes the carbon dioxide to dissolve into the aqueous medium and form carbonic acid. The formation of carbonic acid and, in the case of carbohydrate type wastes, the formation of acetic acid, may cause the pH to drop within the reactor vessel requiring the addition of caustic soda to maintain the reactor at a normal operating pH of 6.8 or greater. The absence of buffering in high strength wastewater creates a constant demand for caustic soda. The addition of caustic soda can be the most expensive feature of operating an anaerobic process.
A downflow reactor design may further contribute to a build-up of acidic conditions within the reactor. The continued metabolism of organic material may contribute to a pH gradient within the vessel. Areas toward the bottom of the reactor, under the influence of pressure due to the depth of the liquid, and away from any means to vent carbon dioxide, may have greater concentrations of carbonic acid.
The problem of acid formation within the reactor vessel is further compounded when the wastewater feed is acidic in itself. Indeed, several industrial processes may produce organic acids and other organic wastes of limited solubility in the wastewater stream in which they are suspended. By way of example without limitation, the production of terephthalic acid for use in the plastic industries results in a waste containing acetic acid, benzoic acid, terephthalic acid, isophthalic acid, orthophthalic acid, p-toluic acid, trimellitic acid, 4-carboxybenzaldehyde and hydroxymethylbenzoic acid. Some of these aromatic acids may exist in waste streams in concentrations greater than their solubility. When present, aromatic acids may cause an acidic feed and must be neutralized prior to degradation. Entering a reactor having a low pH, the aromatic acids may precipitate from the solution reducing the effectiveness of the anaerobic process in metabolizing the organic constituents of the wastewater stream.
The areas toward the bottom of the reactor, with the high levels of carbonic acid, are potential sources of base with the removal of the carbonic acid. Thus, assuming that the contents of the reactor are buffered to an approximately neutral pH, removal of carbonic acid or carbon dioxide will produce a basic liquor available to neutralization incoming acid feed.
Further, the raw feed from many industrial processes is unsuitable for biological decomposition due to temperature constraints. In a typical process, feed from industrial processes is maintained in retention ponds to provide a steady pool or reservoir of feed for the anaerobic process. The retention ponds are further equipped with aeration means, including sprayers or descending baffles, in order to bring the high temperature raw feed from industrial processes to substantially ambient temperature.
The control of temperature and pH in anaerobic processes is energy intensive and costly. Some upflow reactors appear to recycle the finished liquor from the reactor, which has a substantially neutral pH, to buffer incoming wastewater streams. See U.S. Pat. No. 4,568,464 issued Feb. 4, 1986 to Blay et al.; United Kingdom Patent Application No. 19176/75 published Jan. 26, 1977 to Witt et al.; and "Anaerobic Filter Cuts Costs, Generates Energy" Chemical Week, 42, May 23, 1979.
Some downflow reactor systems appear to use a system of recycle to promote an even distribution of liquid throughout the reactor. U.S. Pat. No. 4,311,593 issued Jan. 19, 1982 to Benjes, et al.
U.S. Pat. No. 4,530,767 to Love, describes a method of air stripping carbon dioxide from an upflow reactor process utilizing a reactor hood.
U.S. Pat. No. 3,981,800 to Ort and U.S. Pat. No. 3,939,066 to Bauer describe a method of stripping carbon dioxide from an anaerobic process utilizing pressure release.
U.S. Pat. No. 3,980,556 to Besik describes a method of air stripping nitrogen gases from the activated sludge and recycle of the regenerated material to the reactor.
U.S. Pat. No. 4,375,412 to Schimel describes a method of withdrawing nitrogen gases under the influence of a vacuum.
However, these references do not teach or suggest methods and apparatus for the anaerobic processing of acid wastes of limited solubility, controlling pH, and controlling the temperature.